Be careful when you use the examples there; it is chock full of errors.
For any OTA that requires a 55mm back focus, and there is a ZWO filter wheel or filter drawer (which presumably has a glass filter, you need to add N mm to the back focus where N is 1/3 of the thickness of the filter.
Two inch Radian and Chroma filters are typically 3mm thick and the back focus needs to be 56mm and not 55mm.
See page 8 of Katie Schwertz' Master's thesis:
http://www.loft.optics.arizona.edu/documents/journal_articles/2010_Katie_Useful%20Estimations%20and%20Rules%20of%20Thumb%20for%20Optomechanics.pdf
Chen
